1. Field of the Invention
This invention relates to a belt type continuously variable transmission in which a belt is wound around a primary pulley and a secondary pulley, each of which is composed of two sheaves and, more particularly, to a continuously variable transmission, suitable for a transmission system for an automobile, specifically, in which the belt type continuously variable transmission and a planetary gear unit are combined.
2. Description of Related Prior Art
Automatic belt type continuously variable transmissions have drawn attention in response to demands for improvement in fuel efficiency and driveability.
Transmission systems in which a belt type continuously variable transmission (hereinafter "CVT") and a planetary gear unit are combined to augment gearshift ranges have been disclosed in the prior art, for example, in Japanese Patent Laid-Open Application No. 59-110,954. In such a transmission system, the gear ratio of the power transmission path or mechanism, such as the planetary gearing, can be set so that the CVT delivers zero output, as a whole, at a certain speed-change (pulley) ratio. In such a case, where the CVT is controlled to operate with a target speed-change ratio (target pulley ratio), the CVT does not theoretically transmit any torque (either forward or reverse) from the engine to vehicle wheels, i.e. it enters into a non-drive state with the manual clutch disengaged, wherein the engine brake does not operate. During such a nondrive state, however, the output torque varies greatly when the CVT deviates even slightly from the target pulley ratio, because the output torque ranges from infinity to zero. It is therefore difficult in a practical sense to produce the non-drive state by setting the target pulley ratio as a single point through changing and controlling the axial forces on the pulleys of the CVT.
In the CVT disclosed in the above publication, the gear ratio of the power transmission path, i.e. the gear ratio of the overall transmission system, is set to a value excluding zero such as 0.4 to 4, and the transmission is equipped with a torque converter with a direct coupling clutch (drive starter device) and a clutch brake for switching between normal (forward) and reverse rotation (normal and reverse rotation switching device).
With such a CVT, during operation with forward rotation, the engine delivers the engine torque to the primary pulley of the CVT through the torque converter and the clutch for forward rotation, as well as to a rotary gear element, e.g. sun gear, of the planetary gear unit, and the planetary gear unit thereby amplifies the output rotation of the CVT and delivers it to a differential device. During the reverse rotation, the normal and reverse rotation switching device cuts off the power transmission to the CVT, thus resulting in a stop state for the CVT. The rotation is transmitted to a rotary element or elements of the planetary gear unit through the torque converter to output the reverse rotation, with the CVT in the stop state.
The above-described CVT, however, requires a forward/reverse rotation switching device and a drive start device, such as a torque converter, and is thereby disadvantaged in terms of costs. The requirement for such devices also limit the degree to which the CVT can be made compact in size and, accordingly, its mountability in an automobile.